The present invention relates to implants for repair of spinal defects and injuries. It relates in particular to implants having utility to maintain intervertebral spacing after disc removal. It further relates to intervertebral spacers which can permit at least some remaining spinal flexibility.
Natural intervertebral spinal discs serve multiple purposes. The first, and perhaps most basic, is to preserve the anatomical spacing between adjacent vertebrae such that the spinal column and horizontally branching nerve bundles are allowed to function normally. A second major purpose is to allow relative flexural motion between adjacent vertebral bodies. Natural flexibility, at least of the upper spine, permits rotation about two horizontal axes and one vertical axis. Finally, the disc also serves as a shock absorber for columnar loading of the spine. However, mechanical trauma, degenerative disc disease, and other pathogenesis can compromise a disc""s ability to maintain the spacing, facilitate flexibility, and provide the shock absorbing function.
One option after the surgical removal of a damaged disc might be to allow the intervertebral space to collapse. Collapse, however, usually causes compressive damage to the spinal cord and the associated nerve bundles. Also, the intervertebral space may fill with scar tissue that can aggravate nerve damage. Thus the simple removal of a damaged disc, without providing for maintenance of the normal vertebral spacing is undesirable.
Discectomy therefore is usually supplemented by (1) rigid fusion of the adjacent vertebrae with new bone growth; (2) application of rigid nonfusing hardware; or (3) insertion of an artificial disc. As the patent literature shows, these approaches have been tried in many variations but have significant disadvantages.
a. Vertebral Fusion
Well-known to those skilled in the art is interbody fusion wherein the disc is partially or wholly excised and living bone or various forms of calcium phosphate or other substrate material is placed within the space previously occupied by the disc for the purpose of restoring the normal separation, strength, and stability. Fusion is intended to provide healthy, living bone between the adjacent vertebrae.
For fusion to occur within the disc space, it is frequently necessary to prepare the adjacent vertebrae by excising portions of the cortical exterior surfaces, usually of the vertebral endplates so as to allow an interposed bone graft to come into direct contact with the cancellous (and vascularized) interior of the vertebrae.
Historically, two types of bone grafts have been available for anterior cervical fusion. One type is a round dowel plug-like configuration, such as the original Cloward dowel. The second type, a rectangular plug in a rectangular hole, is often referred to as the Smith-Robinson technique. The bone can be obtained from the patient""s own iliac crest, from allograft cadaveric crest, or cut from a hard femoral cortical bone.
Both techniques can have disadvantages. For example, the dowel approach frequently flattens the intervertebral space, causing it to induce an undesirable kink in the natural lordosis. Also, the instrumentation used to implant the dowel is bulky and tends to be difficult to use. Thus many surgeons have abandoned this technique. The Smith-Robinson approach has problems because lack of preparation of the endplates leads to non-incorporation and eventual collapse. Moreover, the use of autograft bone to fill the disc space is often less than optimal since it requires an additional incision and healing and is of limited availability in its most useful form.
b. Artificial Discs
Extensive research has been devoted to developing an effective artificial disc. No such device has yet been found that is medically acceptable. Examples of various prosthetic or artificial disc attempts include Fassio, U.S. Pat. No. 2,372,622; Stubstad, U.S. Pat. No. 3,867,728; Froning, U.S. Pat. No. 3,875,595; Patil, U.S. Pat. No. 4,309,777; Kuntz, U.S. Pat. No. 4,349,921; Hirayama, et al., U.S. Pat. No. 4,946,378; Nishijima, et al., U.S. Pat. No. 5,899,941; Bryan, et al., U.S. Pat. No. 6,001,130. Substantial numbers of others exist.
c. Spacers
A number of intervertebral spacers designed for installation between adjacent vertebrae have been patented. These include: U.S. Pat. No. 6,086,613 to CAMINO, et al., relates to intervertebral spacers used in connection with spinal fusion. U.S. Pat. No. 6,039,762 to McKay describes a spacer body having an outer surface and a height approximating the height of a human disc space, said body composed of a porous, biocompatible ceramic material for permitting tissue ingrowth therethrough. U.S. Pat. No. 5,989,291 to RALPH, et al. describes an intervertebral spacer having a pair of opposing plates for seating against opposing vertebral bone surfaces, separated by at least one spring mechanism. In a first embodiment there are two Belleville washers which are oriented in opposite directions such that the narrow ends thereof are in contact with each other and the wider ends are in contact with the respective end plates. The Belleville washers work as springs. (Belleville washers are described as frustoconical in shape.)
U.S. Pat. No. 5,984,922 to McKay describes a spinal fixation device consisting of an intervertebral body wedge which is inserted between vertebral bodies by sequentially expanding the disc space using a spacer. No posterior fixation devices are needed for stabilization of the vertebrae. U.S. Pat. No. 5,888,222 to COATES, et al. describes a spinal spacer that engages adjacent vertebrae. The spacer includes a body having two ends, at least one of which xe2x80x9cdefinesxe2x80x9d an inferior vertebral engaging surface having a set of migration resistance grooves. U.S. Pat. No. 5,534,029 to Shia discloses an articulated vertebral body spacer consisting of a pair of upper and lower joint pieces to be inserted between vertebrae. This spacer allows its components to pivot in accordance with forward/backward and bending motions of vertebral bodies. U.S. Pat. No. 4,599,086 to Dogy discloses a spinal stabilization device positionable between separated but neighboring vertebrae. A wrench can be inserted into the opening until it engages a pair of pins, and then the wrench can be rotated to advance the pins into the abutting vertebral bodies.
Two spacer inventions that are designed specifically for use in connection with subsequent fusion processes are: U.S. Pat. No. 5,888,223 to BRAY, Jr., which discloses an anterior fixation device consisting of an oval shaped hollow intervertebral spacer and a retaining plate. The spacer has a side wall and superior and inferior walls. The superior and inferior walls are dome-shaped and porous to allow bone to grow through the device to achieve fusion of two adjacent vertebral bodies; and U.S. Pat. No. 5,865,848 to Baker describing a spinal fusion implant assembly for spacing vertebrae. The device includes a translation mechanism for providing relative motion between the components.
An additional area of prior art includes all devices designed to be applied to one or more exterior surfaces of the spine. Such devices include all types of plates, struts, and rods which are attached by hooks, wires and screws. Other devices are simply variations on the use of rods (e.g. Harrington, Luque, Cotrel-Dubosset, Zielke), wires or cables (Dwyer), plates and screws (Steffee), or struts (Dunn, Knowles).
A preferable spacer would at least to some extent preserve the ability of the patient to rotate portions of the spinal column about two perpendicular horizontal axes (and linear combinations thereof) and about the vertical axis. All of the prior art approaches suffer from the drawback that they either do not attempt to preserve such flexibility or they are unable in practice to achieve a workable device. In addition they suffer from other drawbacks.
The principal drawbacks of the fusion approach are that until very recently the success rate, that is, actual achievement of fusion, has been very modest. Secondly, in cervical as opposed to lumbar application, the diminution in mobility of the spine about rotational axes is a much more significant lasting effect for the patient.
The principal aim of artificial discs has been to provide a prosthesis which preserves as much of the patient""s prior ability to move as possible. The principal drawback of artificial discs is that they simply do not work in a medical sense.
Spacers have the drawback that, by themselves, they do not achieve the beneficial results of fusion. On the other hand, most of them are rigid, and therefore also have the same drawback as successful fusion, namely reducing the patient""s flexibility.
Accordingly, it is an object of the present invention to provide a single-piece intervertebral implant that can serve as a spacer between adjacent cervical vertebrae. Another object of the invention is to provide an intervertebral implant that allows relative motion of the adjacent vertebral bodies. Another object of the present invention is to provide a low cost intervertebral implant that is small enough to allow rapid and easy anterior or posterior installation into either the cervical or the lumbar regions of the spine. Another object of the present invention is to provide an intervertebral implant device that is self-adhering upon installation. Another object of the present invention is to provide an intervertebral implant device that can maintain intimate, grasping adherence with the vertebral bodies, even when loaded in tension. Another object of the present invention is to provide an artificial disc prosthesis able to allow for full bending movement of the two vertebral bodies between which it is installed, allowing flexibility within both the lateral and sagittal planes. Yet another object of the present invention is to provide an intervertebral implant device that can pierce the bony cortex of the vertebral bodied during installation. And yet a further object of the present invention is to provide an intervertebral implant device that provides self-centering alignment upon installation.
Additional objects of the invention are to provide a single-piece spacer device which alone, or with a second such spacer device, can serve as a spacer between adjacent vertebra within the lumbar region of the spine as part of a vertebral fusion process. Yet another object of the present invention is to provide an intervertebral implant device that provides the correct amount of loading in order to promote bone growth and vertebral fusion A final object of the invention is to provide a single-piece device which alone in the cervical region of the spine or in combination with a second like device in the lumbar region of the spine can be driven into an intervertebral space with a single force-applying operation for each of the one or two devices.
The present invention is an implantable intervertebral spacer designed to both preserve the space between adjacent vertebrae and to provide some continued spinal flexibility by virtue of its flexible construction. This spacer is also characterized by single piece construction and by the property of being self guiding upon initial insertion into the intervertebral space. This invention also has four spaced-apart tail pieces flexibly linked to one another, consisting of one upper set of two horizontally spaced-apart tail pieces and one lower set of two horizontally spaced-apart tail pieces, the upper and lower sets also being vertically spaced-apart from one another. Each of the two upper spaced-apart elongated tail pieces is independently flexibly linked to each of the corresponding spaced apart elongated lower tail pieces. Each of the tail pieces also comprises an elongated post supporting a bone gripping barb and a cutting edge having an inwardly angled facet. Upon installation between adjacent vertebral bodies, the implant is self locking by virtue of strain induced in the elongated barb-supporting posts. The device is implantable as a single unit within the intervertebral space between adjacent cervical vertebrae or it is implantable in conjunction with a second spacer within the same intervertebral space. Because of the cutting edges and barbs, it is implanted by being impacted with a driver device, at which point it is firmly seated and will not pull or creep out. The device also has a wedge shaped front portion to guide it into the desired position during installation. The bone gripping induced strain manifests within each of the elongated posts supporting each barb of each spaced-apart pair of barbs when the facet causes inward bending strain of the elongated posts upon installation into the vertebral bone.
The present invention is an implantable intervertebral device that can be configured also to promote bone growth and vertebral fusion. The vertical slot between the upper and lower sets of the two horizontally spaced-apart tail pieces can be loaded with bone graft material and/or bone morphogenic protein so as to create a fusion column and promote intervertebral fusion. It is known that compressive loading of graft material promotes new bone growth and healing. (This principle is generally referred to as Wolff""s law.) The horizontal slot between the upper and lower sets of tail pieces can be varied in height, and the radius of the internal void in the portion connecting the upper part of the device to the lower part of the device can be varied, both so as to provide flexing sufficient to produce enough loading on the graft material to promote bone growth and vertebral fusion while still providing sufficient compressive strength to maintain the necessary intervertebral spacing.
The spacer of the current invention can be fabricated from metal, preferably titanium or stainless steel. Or it can be fabricated from organic polymer such as thermoplastic or thermosetting plastic unfilled or filled with fibers such as glass, metal or carbon. Preferably, said implant spacer, if fabricated from organic polymer, is to be fabricated from polyether ether ketone (PEEK), most preferably filled with fibers of metal or glass or, most preferably, carbon.
A second embodiment of the said cervical lumbar implant spacer employs the use of bullets situated upon elongated posts instead of barbs as a way to grip the vertebral bone. The forward cutting edge of each bullet has an angled facet that causes induced strain to arise within the elongated support post during installation.